All this activity is the result of Italy being right in the thick of the slow collision between the African and Eurasian plates that has, amongst other things, resulted in the uplift of the Alps. At first glance, it is therefore quite surprising to see that the focal mechanism for this earthquake is characteristic of an extensional earthquake, due to stretching of the earth’s crust, not a compressional one. The extension is oriented in a northeast-southwest direction, at right angles to the Apennine range:

So why is extension occurring in a mountain range? The tectonic history of the western Mediterranean is actually quite complicated; rather than being the last remnants of a large ocean that has been mostly destroyed by subduction as Africa and Europe move together, the oceanic crust here has actually all been created by back-arc spreading in the last 40 million years or so, as the collision zone (marked by the red line which runs around the east and south-east coast of Italy and Sicily and into North Africa), has migrated south and east away from Europe, stretching out the crust in the over-riding plate as it does so.

So although at a broad regional scale two plates are colliding, at a more local level the current back-arc spreading in the Tyrrhenian Sea to the southwest appears to be a major driver of tectonics in Italy, to the extent that thrust faults that built up the Apennines are now being reactivated as extensional normal faults. Unfortunately, this switch doesn’t make the earthquakes themselves any less damaging when they do occur.

Thanks for this, from a non-expert. Now that you’ve told me that the Tyrrhenian Sea is spreading, everything else follows easily. But I’d like to put in one vote for a follow-up post or further explanation: I don’t understand what forces would drive the Tyrrhenian Sea to spread, especially given the broader picture of the African-European collision. Why should spreading behind a big trench occur, in general and in this particular case?
Should I picture a spreading ridge down there that’s like a miniature version of the Mid-Atlantic Ridge, locally strong, but globally overwhelmed by even more powerful spreading elsewhere that drives the continents together?

Kim has a post up which nicely explains the basics of trench “roll-back”. Basically, there is a vertical component to subduction; the slab is sinking downwards into the mantle, which can create space for the over-riding plate to spread outwards into. It’s not a perfect analogy, but think of the way that a waterfall cuts back into the bedrock – the flow of water stays the same, but its physical location is moving backwards.
Another process which might lead to extension in the overlying plate is the mantle beneath it being dragged towards the subducting slab as it descends – a sort of suction force often referred to as “corner flow”.

I have to add another important mechanism of ridge-normal extension named gravity-driven collapse of an orogen. It is produced by gravitational energy of high mountain range supported by light crustal roots in the relatively heavy mantle. This factor is common in all elevated areas, especially those, isostatically balanced. It can be the matter of discussion (or modeling), which factor is dominant, gravity collapse or spreading of the Tyrrhenian Sea.

Chris,
Nice post, and a very clear map on Mediterranean geodynamics. In fact, I might like to use it in the intro presentation for a talk on my geoarchaeology project north of Florence at the Keck Geology Consortium Symposium next weekend. What’s the original source?

nice post. I was about to go to the USGS site to see if the eq was extensional (as I thought it was), but I thought.. nah… I’ll read that blog instead…
The rollback idea for the range goes to the late ’80’s Royden papers, if I’m not mistaken. But seismic tomography suggests a tear, rather than rollback. Independently of that, there’s a suggestion that the extension is driven by the supra-critical wedge (“gravity” if you will) in which case you wouldn’t have the slab rolling back (which would diminish accretion), but rather have it fixed or even moving toward the Aegean… I don’t know what the geodesy suggests, tho’.

I got it backwards in my last comment – I meant to say the supra-critical wedge would imply the trench is rolling toward the Tyrrhenian sea…
another point – its interesting that the focal mechanism is so 60-60. There’s an active low-angle normal fault that likely slipped during this event…

Slightly OT…
I live near enough to have felt the 3.7 under Ulverston on Tuesday 28th. The house shook a bit, then there was a very loud bang and the windows rattled.
What caused the bang? I had no idea that earthquakes made that kind of noise.
(If there is a better blog on which to ask this, please could someone point me in its direction?)

Great explanations and diagrams Chris.
In his new book, The Mounatins of St Francis, Walter Alverez, writes about the Apennines and earthquakes. He describes a hypothesis called delamination and rollback which is he says goes beyond traditional plate tectonic theory.
The delamination and “rapid sinking” of a lower layer of continental crust (under the Apennines which is the upper “floating crust”)causes the compression in front of a moving block and extension behind it. He includes a drawing of a floating block of wood above a tilting tile and how the movement of liquids around the floating block would send the block toward the hinge of the delaminating slab(compression) and extension behind it.
Chapter 14 pages pg 233-238